Method for effecting the rapid heat-treatment of steel plate

ABSTRACT

THE RESPONSE OF STEELS TO THE PRODUCTION OF ULTRA-FINE GRAINS BY A CYCLIC-RAPID AUSTENITIZATION IS ENHANCED BY THE PRECONDITIONING TREATMENT OF THIS INVENTION. THIS PRECONDITIONING IS ACCOMPLISHED BY HEATING THE ARTICLE TO A TEMPERATURE JUST BELOW THE A1 TEMPERATURE OF THE STEEL, FOR A TIME OF ABOUT 5 TO 30 MINUTES.

United States Patent Olfice 3,692,591 Patented Sept. 19, 1972 US. Cl. 148-444 3 Claims ABSTRACT OF THE DISCLOSURE The response of steels to the production of ultra-fine grains by a cyclic-rapid austenitization is enhanced by the preconditioning treatment of this invention. This preconditioning is accomplished by heating the article to a temperature just below the A temperature of the steel, for a time of about to 30 minutes.

This invention relates to a method for preconditioning steel plates so that they may be effectively heat-treated to produce an unusually fine-grained structure.

It has been shown in US. Pat. No. 3,178,324 that a cyclic heat treatment comprising a rapid austenitization and quench, can effect a substantial grain refinement in steel, thereby resulting in improved strength, markedly lowered ductile-to-brittle fracture-transition temperature, and little loss in notch toughness at temperatures above the fracture transition. This grain refinement results from the fact that a fast heating and cooling permits the ferrite to austenite transformation to predominate over the slower, diffusion controlled austenite grain growth process, thus making it possible to retain in the final product an austenite grain size closely related to the prior ferrite grain size. Similarly, it would be expected that a very fine dispersion of carbide particles in the ferrite matrix would result in more nucleating centers for the formation of new austenite grains, since austenite is known to nucleate at carbide-ferrite interfaces (see for example, Speich et al., Trans. Met. Soc., A.I.M.E., vol. 245, p. 1063, 1969). Thus, greater grain refinement and a greater improvement in physical properties would be expected from rapid austenitization of a steel with a fine carbide dispersion than the same steel with larger agglomerated carbides. However, contrary to this expectation, it has now been found that when such rapid heat treatment is conducted on a relatively coarse carbide structure, such as an ashot-rolled starting structure, an improved strength-toughness relation is achieved as compared with a similar rapid heat treatment conducted on a fine carbide starting struc ture, e.g. the as-quenched bainitic or martensitic structure which would result from the fast cooling as taught in the above patent. Thus, significant improvements in strength and ductility can be achieved if, prior to each of the rapid heating cycles as employed in US Pat. No. 3,178,- 324, the steel is preconditioned by the method of this invention, to provide a desirable coarse carbide structure.

While uniform rapid heating can be achieved in thin plates employing hot salt or lead baths, these heating methods are not effective for steel plates in the range of /2 inch to 3 inches in thickness. At first thought it would appear that induction heating would be practical for these thicker plates, since in this method the heat is generated internally, therefore heating of the midthickness of the plate would not be totally dependent on the rate of heat conduction from the plate surface. However, uniform through heating by induction, has the following disadvantages: (a) the depth penetration of the generated heat is dependent on the frequency of the induction current. Therefore, since variable frequency sources, at the power levels required, are not commercially available, and since the optimum frequency for maximum heating at the midthickness of a plate is different for each plate thickness; in a practical commercial system, a compromise frequency would be required, e.g. one that offers reasonably uniform heating for the various thicknesses employed; (b) the depth of penetration of a given frequency (shown below) is much lower below the curie temperature of the steel, than above the curie temperature (about 1250 F.)

Penetration in inches Below Above Frequency curie curie (hertz) temp. temp.

so that the compromise frequency must also take this factor into account. It has now been found that the required fast heating rate is only critical above the A temperature, so that the above disadvantages of non-uniform heating may be substantially overcome by a preheat below the A temperature, followed by the necessary rapid heating to above the A temperature of the steel.

It is therefore an object of the instant invention to provide a method for enhancing the response of steel to rapid austenitization, when employing a cyclic heating process for producing fine grained steel.

It is another object to provide a practical and commercial method for effecting uniform cyclic heat treating of heavy plate sections, tubing and various other massive structures.

These and other objects will become more apparent from the following specification and drawings in which:

FIG. 1, A, B, C, and D illustrates the effect of initial grain structure on the degree of grain refinement which can be achieved after only one cycle of rapid austenitization.

FIG. 2 is a graph illustrating the effect of preconditioning time on the physical properties of a cyclicly heattreated steel.

In an experiment leading to the instant invention, the effect of initial grain size on response to rapid austenitization was noted in comparing the effectiveness of a single cycle of rapid heat treatment on both a coarse grained and a fine grained carbon steel. As stated hereinabove, one Would expect that a steel made to fine grain practice (e.g., one in which the presence of aluminum nitride particles prevent the growth of austenite grains) would exhibit a finer prior austenite grain size, particularly after only one cycle of rapid austenitization. However, the results of the experiment below, show that the degree of grain refinement achieved (FIG. 1) and its concomitant effect on mechanical properties (Table I), was significantly greater for the semi-killed coarse grained steel.

Two sets of /2 inch steel plates, one set prepared to coarse grained practice, the other employing fine grained practice (see Table I, for chemical composition) were conventionally heated to 1675 F., held for /2 hour and air cooled, to simulate the conventional normalized condition. Two similar sets were preheated to 1300 F., and rapidly austenitized at 16 F./sec. to 1580 F. and likewise air cooled. The effect of significantly greater degree of grain refinement for the coarse grained steel is especially evident in the marked increase in yield strength and lowering of the ductile fracture temperature. It would therefore appear, that the effectiveness of rapid austenitization could be enhanced by a preconditioning treatment which provides a particularly responsive structure.

TABLE I.CHEMICAL COMPOSITION C Si Al ll/Ill N 1 Coarse grained 0. 24 0. 04 0. 001 1. l9 0. 005 Fine grained 0. 24. 0. 45 0. 048 0. 94 0. 007

RELATIVE INCREASE IN PHYSICAL PROPERTIES OVER SIMILAR CONVENTIONAL AUSTENIIIZATION Fine grained 2.4 percent 2.5 percent A 35 F.

To illustrate the effect of the preconditioning treatment of this invention, a 1-inch thick plate of a 5 Ni-Cr-Mo-V steel was rapidly austenitized (see Table II for chemical analysis) at 16 F./second to 1500 F. and then Water quenched. The cycle was repeated five times (e.g. the method of US. Pat. No. 3,178,324) after which the plate was tempered at 400 F. for one hour. The mid-thickness physical properties of the resultant product are reported in FIG. 2, at time. A series of the same steels were similarly rapidly austenitized at 16 F./ second to 1500 F., water quenched and reheated for five cycles, except that prior to each rapid austenitizing treatment, the plates were preconditioned at 115 0 F. for various time periods. While all periods (within the reported times) of preconditioning would appear to effect at least some enhancement in properties, it may be seen in FIG. 2, that preconditioning for a time of about to 30 minutes will provide a significant increase in strength and ductility. Although conditioning for longer than about 30 minutes will not seriously decrease properties, such longer times are less desirable in that they are uneconomical.

TABLE IL-CHEMICAL lgMPlOlqSITION OF 5 Ni-Cr-Mo-V In addition to enhancing the response of steel to a rapid austenitization treatment, the preheat treatment of this invention will also aid in eliminating excessive temperature differences between the midthickness and the surface of heavy plate or pipe sections. Steel plates (see Table II, for chemical analysis), two inches thick were rapidly austenitized (without a preheat) by induction heating at a frequency of 960 c.p.s. (this frequency represents the best compromise for midthickncss heating of plates from /2 through 3 inches thick). As shown in Table III below (Example a), when the surface of the plate was heated to optimum peak temperature, the temperature of the midthickness of the plate was below the A temperature (1415 F. for this composition) of the steel. Thus, the midthickness was not completely austenitized and therefore did not achieve full grain refinement, resulting in a smaller improvement in yield strength than the plate surface. Likewise (Example b), when the midthickness was heated to optimum peak temperature, the surface temperature became so high that grain coarsening resulted, along with its consequent loss in yield strength.

In comparison, a similar 2-inch thick plate was induction heated to a temperature of 1100 F.; the induction power was then reduced and the plate held at temperature until the temperature of the center of the plate was equalized with the surface temperature. The times required for this equalization will vary from about 30 seconds to about minutes for plates within the range of about to 3 inches thick. After temperature equalization was achieved, full power was again applied to the induction coil so that the center of the plate was heated to above A temperature. On reaching temperature, the plate was water quenched. As may be seen in Table III (Example c) the preheat makes possible the achievement of near optimum temperatures both at the surface and midthickness, thereby resulting in a much more uniform plate product.

TABLE III.5 CYCLE HEAT TREATED 2-INCH THICK PLATES OF TABLE II WITHOUT A TEMPERATURE EQUALIZING PREHEA'I V-notch 0.2% Reducemergy Peak ofiset tion of absorb.

temp, Y.S 'I.S., area, at F.,

Ex. Location F. K s 1 K si. percent t.-lb.

( {Surface 1, 446 203 68.0 62

a Midthickness 1, 354 163 197 63. 6 74 (b) {Surface 1, 737 167 193 65. 7 67 Midthickness- 1, 479 173 58. 2 44 Surface (c) '{h/Iidthickness. 20s

The preconditioning treatment of the present invention will therefore provide a plate product with substantially uniform properties throughout the thickness, a uniformity which would otherwise be unobtainable employing the fast heat-up rates (from room temperature) re quired for effective grain refinement.

We claim:

1. In a method of producing ultra-fine grains in a steel product, which method consists of employing at least one cycle in which the steel is rapidly heated to above its A temperature for a time just sutficient to transform it to an austenitic structure, and is then immediately cooled at a rate equal to or greater than that required to transform the austenite to a fine grained ferrite-pearlite aggregate, the improvement which comprises preconditioning said steel by preheating at a temperature in the range of from (A -150) F. to a temperature just below the A temperature of the steel, for a time of about 5 minutes to about 30 minutes, prior to each of the rapid heating cycles employed.

2. In a method of producing ultra-fine grains in steel products having a thickness of from about /2 inch to 3 inches, which method consists of employing at least one cycle in which the steel is rapidly heated to above its A temperature for a time just suflicient to transform it to an austenitic structure, and is then immediately cooled at a rate equal to or greater than that required to transform the austenite to a fine grained ferrite-pearlite aggregate, the improvement which comprises preconditioning said steel by preheating the steel at a temperature in the range of from (A -150T F. to a temperature just below the A temperature of the steel, for a time at least sutficient to substantially equalize the temperature of the surface with that of the midthickness of the product, prior to each of the rapid heating cycles employed.

3. The method of claim 2, wherein only one cycle is employed which consists of (a) said preconditioning below the A temperature; (b) said fast heat-up to just above the A temperature; and (c) said cooling to transform the steel.

References Cited UNITED STATES PATENTS 3,178,324 4/1965 Grange et al 148-l44 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

